In underground electrical distribution systems that are energized to, e.g., 15 kV to 35 kV, it is common to employ high-voltage connector assemblies of the elbow/bushing variety. The IEEE STD 386 standard covers such electrical connectors. In their earliest and most basic form, bushing inserts had a squared-off shoulder with no venting and no latch indication, where the shoulder of the bushing is the area where the cuff of the elbow fits against the bushing. Oftentimes, bushing/elbow assemblies allow for connection and disconnection when the line is carrying current (i.e., loadmake and loadbreak operations).
FIG. 6 is an illustration of terminator/bushing assembly 600, which is one prior art embodiment. Assembly 600 includes elbow terminator 610 and bushing insert 620. Elbow terminator 610 includes sleeve 612, cuff 611, and probe 613. When latched, sleeve 612 fits over bushing insert 620 such that the inner surface of cuff 611 fits snugly up against shoulder 621, and probe 613 is received into conductive tube 622. In FIG. 6, terminator 610 and bushing insert 610 are not drawn to the same scale.
At 25 kV there have been problems in the industry for many years concerning a phenomenon known as partial vacuum-induced flashover. Rarely, when an operator would pull an elbow off of a bushing, there would be an arc from the exposed conductive insert (of the elbow) to a conductive grounding shield on the bushing. It was discovered that flashover is caused by a decrease in the dielectric constant of the air trapped in the assembly due to a partial vacuum during loadbreak operations. In IEEE STD 386 elbow/bushing assemblies, the cuff of the elbow overlaps the collar of the bushing by about ½ inch, so that the first ½ inch of travel during a loadbreak operation creates a volume inside the elbow-bushing interface connection. The volume of air becomes greater without letting any other air enter the assembly, thereby lowering the pressure of the air. When air pressure is lowered, the dielectric strength of that air is also lowered, as described in Paschen's curve. The lowered dielectric strength of the air leads to lowered resistance and sometimes, arcing.
One prior solution to the flashover problem includes the use of additional insulation in the elbow terminator. Such a solution is described in U.S. Pat. No. 5,655,921, which is incorporated by reference herein. Furthermore, U.S. Pat. No. 5,655,921 also shows the use of an insulating layer placed over a grounding shield to prevent flashover.
Yet another approach includes decreasing or relieving the partial vacuum as it occurs. One such solution uses a vented bushing insert, which has slots and grooves on its shoulder to allow air to go underneath the cuff of the elbow and relieve the air pocket that is between the cuff of the elbow and the shoulder of the bushing. One problem with that design is that it only vents one of the cavities in which the vacuum is created, while leaving other small cavities unaddressed, e.g., the areas around the nose of the bushing.
Another problem with vented bushings is that the vents get plugged up with grease. When linemen put elbows and inserts together, they typically use silicone lubricants to slide the two rubber pieces together. It is an interference fit that is very tight, and the lubrication makes the elbows operable over the next twenty to thirty years. Over time, the lubrication thickens up, turns gluey, and will clog up the vents, making the elbow harder to operate, and pulling more vacuum. More vacuum leads to a greater chance of flashover. An example of a vented shoulder is shown in U.S. Pat. No. 6,939,151.
The difference in performance between the insulated elbow solution and the vented bushing solution led to changes in the IEEE standard for testing bushing elbow assemblies. The OIACWT, (Operating Interface AC Withstand Test) provides a way for testing new elbow/bushing designs. There are two tests in the standard—Option A and Option B. Option A is a partial vacuum test at 27.5 kV, with no lubricant, and Option B is a partial vacuum test with aged lubricant at 30.5 kV.
A beveled insert is the focus of another solution technique. A beveled insert refers to a bushing insert where the shoulder of the bushing is chamfered. Usually, the shoulder of a bushing is a ninety-degree corner per the IEEE STD 386 standard, but in a beveled insert, the corner is at a much shallower angle, e.g., forty-five degrees. The shallower angle keeps the cuff of the elbow from sealing to the shoulder of the bushing, thereby preventing partial vacuum from occurring. In order to further reduce cuff/shoulder sealing, some beveled inserts include flange-like protrusions that extend radially outward from the beveled surface.
Yet another solution includes using a J-ring adjacent to the shoulder of the bushing to relieve the partial vacuum at a short travel distance of the cuff. An example of a J-ring solution is shown in U.S. Pat. No. 7,083,450, which is incorporated by reference herein. J-ring solutions attempt to prevent cuff-shoulder sealing by changing the geometry of the outside surface of the bushing so that the cuff cannot create a seal during loadbreak. The J-ring design is similar to a counterbore design with an added protrusion, an example of which is labeled 115 in FIG. 3 of U.S. Pat. No. 7,083,450. The protrusion prevents the tip of the cuff from sealing along the bottom shelf of the counterbore. Once the tip of the cuff clears the point of the protrusion, it allows air to flow around the cuff of the bushing, thereby relieving any partial vacuum.
It is important to note that the J-ring design relieves vacuum differently from the other designs. Vented shoulders and beveled inserts hold the cuff outward to allow air to go underneath the cuff, whereas a J-ring design allows the cuff to fall. Typically, J-ring designs do not succumb to grease pack like vented shoulders do. Further, because so much material is taken away from the insert due to the counterbore, the starting volume of trapped air when the elbow is mated to the insert is much greater than that of the beveled insert and the vented insert. Thus, the pressure drop is not as severe, simply because the starting volume in the steady state latched position is so much greater than the general design. Thus, J-ring solutions provide better vacuum-relieving properties than other currently-available solutions.